In holographic data storage, a data-bearing object wave is overlapped with a reference beam inside a photosensitive media. Typically, the interference pattern generated by the object and reference beams modulates the material index of refraction throughout the media resulting in a phase hologram. Holographic storage systems are storage systems that use holographic storage media to store data. Holographic storage media take advantage of the photorefractive effect described by David M. Pepper et al., in “The Photorefractive Effect,” Scientific American, October 1990 pages 62-74. The index of refraction in photorefractive materials can be changed by light that passes through them. By controllably changing the index of refraction in such materials, information can be stored in the photorefractive material in the form of interference patterns (or holograms). Holographic storage systems allow for high-density, high-capacity, and high-speed storage of information in holographic storage media.
The photorefractive effect occurs in specific types of crystals (most commonly seen in iron-doped lithium niobate). Another class of storage materials is photopolymers (light sensitive polymers). Photopolymers can also record phase holograms, but they do not do so via the photorefractive effect.
A hologram stores data in three dimensions and reads an entire page of data at one time, which is unlike an optical CD disk that stores data in two dimensions and reads a track at a time. The advantages of recording a hologram are high density (storage of hundreds of billions of bytes of data), high speed (transfer rate of a billion or more bits per second) and ability to select a randomly chosen data element in 100 microseconds or less. These advantages arise from three-dimensional recording and from simultaneous readout of an entire page of data at one time.
A hologram is a collection of patterns, also known as gratings, which are formed when two laser beams interfere with each other in a light-sensitive material (LSM) whose optical properties are altered by the intersecting beams. Before the bits of data can be imprinted in this manner in the LSM, they must be represented as a pattern of clear and opaque squares on a display such as a liquid crystal display (LCD) screen, a miniature version of the ones in laptop computers. A laser beam, for example, is shined through this crossword-puzzle-like pattern called a page, and focused by lenses to create a beam known as a signal beam. A hologram of the page of data is created when the signal beam meets another beam, called the reference beam, in the LSM. The reference beam could be collimated, which means that its electric field is phase-synchronized, with crests and troughs passing through a plane in lockstep. Such waves are known as plane waves. The grating created when the signal and reference beams meet is captured in the LSM as a modulation of the material's index of refraction.
After recording the grating, the page can be holographically reconstructed by shining the reference beam into the LSM from the same angle at which it had entered the LSM to create the hologram. As it passes through the grating in the LSM, the reference beam is diffracted in such a way that it recreates the image of the original page and the information contained on it. A reconstructed page is then projected onto a detector such as an array of electro-optical detectors that sense the light-and-dark pattern, thereby reading all the stored information on the page at once. The data can then be electronically stored, accessed or manipulated by any conventional computer.
In the typical holographic storage system, two coherent light beams are focused onto a holographic storage medium. The first coherent light beam is a signal beam, which is used to encode data. The second coherent light beam is a reference light beam. The two coherent light beams intersect within the storage medium to produce an interference pattern. The holographic storage medium records this interference pattern by changing its index of refraction to form an image of the interference pattern.
The recorded information, stored as a holographic image, can be read by illuminating the holographic image with a reference beam. When the holographic image is illuminated with a reference beam at an appropriate angle, a signal beam containing the information stored is produced. The appropriate angle for illuminating the holographic image will be the same as the angle of the reference beam used for recording the holographic image. Small deviations in the reference beam angle during readout will cause the hologram to not reconstruct. This allows many such holograms to be multiplexed in the same volume of material by using a densely spaced set of reference beam angles to record the holograms. Accordingly, high storage capacity can be obtained since the same volume can be used to store multiple holographic recordings.
Information can be encoded within the signal beam in a variety of ways. One way of encoding information into a signal beam is by passing a beam of light through an electronic mask, called a spatial-light modulator (SLM). The SLM is a two dimensional matrix of squares. Each square in the matrix can be directed to transmit light, corresponding to a binary 1, or to block light, corresponding to a binary 0. Equivalently, an SLM can be a reflective device, such that a pixel representing a binary 0 fails to reflect the incoming light, while a binary 1 strongly reflects the light. The signal beam once encoded by the SLM is focused onto the storage medium, where it intersects with a reference beam to form an interference pattern. The volumetric interference pattern records the information encoded in the signal beam to the holographic storage medium.
The information recorded in the holographic storage medium is read by illuminating the storage medium with a reference beam. The resulting signal beam is then typically focused onto a sensor such as a Charge Coupled Device (CCD) array or a CMOS camera. The abbreviation “CMOS” is derived from “complementary metal oxide semi-conductor.” The sensor is attached to a decoder, which is capable of decoding the data.
A holographic storage medium includes the material within which a hologram is recorded and from which an image is reconstructed. A holographic storage medium may take a variety of forms. For example, it may comprise a film containing dispersed silver halide particles, photosensitive polymer films (“photopolymers”) or a freestanding crystal such as iron-doped LiNbO3 crystal. U.S. Pat. No. 6,103,454, entitled RECORDING MEDIUM AND PROCESS FOR FORMING MEDIUM, generally describes several types of photopolymers suitable for use in holographic storage media. The patent describes an example of creation of a hologram in which a photopolymer is exposed to information carrying light. A monomer polymerizes in regions exposed to the light. Due to the lowering of the monomer concentration caused by the polymerization, monomer from darker unexposed regions of the material diffuses to the exposed regions. The polymerization and resulting concentration gradient creates a refractive index change forming a hologram representing the information carried by the light.
FIG. 1 illustrates the basic components of a holographic system 100. System 100 contains a SLM 112, a holographic storage medium 114, and a sensor 116. SLM 112 encodes beam 120 with an object image. The image is stored by interfering the encoded signal beam 120 with a reference beam 122 at a location on or within holographic storage medium 114. The interference creates interference patterns (or hologram) that are captured within medium 114 as a pattern of, for example, a holographic refractive index grating.
It is possible for more than one holographic image to be stored at a single location, or for a holographic image to be stored at a single location, or for holograms to be stored in overlapping positions, by, for example, varying the angle, the wavelength, or the phase of the reference beam 122, depending on the particular reference beam employed. Signal beam 120 typically passes through lenses 130 before being intersected with reference beam 122 in the medium 114. It is possible for reference beam 122 to pass through lenses 132 before this intersection. Once data is stored in medium 114, it is possible to retrieve the data by intersecting a reference beam 122 with medium 114 at the same location and at the same angle, wavelength, or phase at which a reference beam 122 was directed during storage of the data. The reconstructed data passes through one or more lenses 134 and is detected by sensor 116. Sensor 116, is for example, a charged-coupled device or an active pixel sensor. Sensor 116 typically is attached to a unit that decodes the data.
The quality of the recorded hologram as measured by such parameters as diffraction efficiency, multiplexing selectivity, and image fidelity is directly influenced by a variety of details specific to each system implementation. However, common to many designs, the reference beam impinges on the photosensitive media at a large angle with respect to the media surface normal. When the reference beam is of a convergent and/or divergent nature, the illuminated spot on the media will be distributed unequally in intensity [Watts/cm2] due to the change in solid angle subtended by different positions on the media surface. The uneven spatial intensity profile results in spatial non-uniformities of the recorded hologram reducing the hologram quality.
Laser beams that are typically used for holographic recording have a spatial intensity profile dictated by the oscillation mode of the laser resonator, with the simplest mode having a Gaussian or bell-shaped profile. Apodization or beam shaping of the reference beam is a technique that redistributes the optical power with the common goal being a uniform intensity distribution.
One method for generating a beam with a uniform (or flat) spatial profile is to simply expand a Gaussian beam and use only the center portion. However, this method trades off the power efficiency to obtain the desired flatness of illumination. For example, if an illumination flatness of 5% is required over a certain area, then typically only 5% of the incident reference beam power can actually be used. It has long been desirable in laser physics to be able to efficiently generate a laser beam with a uniform cross section. Although many ingenious solutions have been proposed, the few that have been implemented suffer from poor flatness, severe diffraction effects and distortion of the wave front, poor quality of the apodized beam, expensive production cost, and high sensitivities to misalignment of the apodizer. In addition, many solutions, including diffractive optics, create a beam that attains uniform intensity in one plane in space, but then diverges and distorts away from that plane.
J. Ashley et al. in “Holographic data storage,” IBM J. Res. Develop. Vol. 44, No. 3, 341 (May 2000), disclose a typical aspherical apodizer. A two-element telescope with transmissive optical elements was designed that produces a relatively flat-top laser beam with the capability of propagating for several meters with little distortion and diffraction-limited wave front quality. The Gaussian-beam-to-flat-top converter utilized a convex aspheric lens to introduce aberrations into the beam and redistributed the laser power from a particular incident Gaussian profile to a relatively flattop profile. A second aspheric optic re-collimated the aberrated beam, restored the wave front quality and allowed it to propagate for long distances without spreading. As a result, only the central 60% of the output power was uniform in intensity to 2%. The input and output beam dimensions are fixed for a given apodizer. FIG. 2 shows an example of input and output intensity profiles (not showing the roll-off) measured using the apodizer of Ashley et al.
Ashley et al.'s method, however, suffers from substantial variation in intensity outside the central 60% of the output power. Therefore, there is still a need for an efficient method for correcting hologram spatial non-uniformities far more tightly to produce a reference beam having a uniform intensity.